The overall aim of this study is to determine the requirements necessary for activation of T-lymphocytes in response to mitogens and antigens. These studies will be undertaken using a unique haplotype-dependent, T-cell mitogen derived from Mycoplasma arthritidis. More specifically, our research is directed toward the following aims: 1) Purification and characterization of the T-cell mitogen derived from M. arthritidis. 2) Accessory cell requirements for presentation of mitogen and antigen to T-lymphocytes and role of soluble mediators. 3) Differential accessory cell requirements for activation of T-cells, T-hybridomas and T-cell clones. 4) Mechanisms of T-cell recognition of processed mitogen and antigens. 5) Mechanisms and gene control of non-MHC factors in mice and rats which suppress responsiveness to mitogens and antigens.